The invention relates to a method for forming electrically conductive circuits on a printed base board with use of copper lamination, and more particularly relates to a method for effectively using an electrically conductive copper paste to form at least two layers of electrically conductive circuits on one side of the base board, to thereby dispense with further processing the base board by forming through-holes therein which may otherwise be required by a conventional method, in which the base board has circuits formed on both sides thereof, so as to electrically connect the circuits to each other. Thus the invention may drastically reduce the production processes, the material required therefor and accordingly to production cost of the product. Moreover the invention may be utilized on a base board to be processed in the conventional method as mentioned above. In this case, the size of the finished product may be remarkably reduced, almost to less than a half of the conventional size.
It is generally known that the electrically conductive circuits formed on a printed base board by copper lamination are often required to be electrically connected. In the conventional method, this has been attained by forming etched copper lamination circuits on both sides of the print base board and then forming electrically conductive holes extended through the base board from one side to the other thereof to electrically connect the circuits on one side of the base board to the circuits on the opposite side of the base board. This conventional method, however, requires processing to provide a copper lamination on both sides of the base board, to etch the copper lamination and then to form a number of holes in the base board by means of a device such as the NC device, thus requiring high material and production costs.
In order to improve the mentioned conventional method, it is preferable to form the electrically conductive circuits in two lamination or layers on one face of the printed base board. In this case, however, it is required to use a specific electrically conductive copper paste having a proper electric conductivity and a proper metal plating capability. However the conventional electrically conductive copper paste is, in contrast to a precious metal such as silver, apt to be oxidized by a heat for hardening the copper paste. Such an oxidized paste will increase the electric resistance and lower the welding capability to the extent that the copper paste is practically useless. Further in order to plate the heated and hardened copper paste, it has been required to activate the surface of the paste by means of a catalyst so as to expose from the resin binder of the paste the copper grains as the nucleuses for plating on the surface of the paste.
The applicant has been for many years in engagement with a work for developing a new electrically conductive copper paste which may eliminate the defects and disadvantages as mentioned above, and has succeeded to realize such an electrically conductive copper paste to be actually used in this field or industry. In particular, these developments include the electrically conductive pastes ACP-020, ACP-030 and ACP-007P of Asahi Chemical Research Laboratory Co., Ltd., which respectively comprise a powdered copper, a synthetic resin and small amount of specific additive such as anthracene. The product ACP-020 includes a main element comprising 80% by weight of powdered copper and 20% by weight of synthetic resin, and is extremely excellent in electric conductivity but may be more or less inferior in the soldering property. The product ACP-030 includes a main element comprising 85% by weight of powdered copper and 15% by weight of synthetic resin, which is excellent in the soldering property but more or less inferior in electric conductivity. The product ACP-007P is an improvement of ACP-030 which is extremely receptive to a metal coating making it possible to apply a metal coating, for example, a chemical coating of copper on a hardened point-like film of this product without the need of using a catalyst.
It is therefore an object of the invention to provide a method for forming at least two laminations or layers of electrically conductive circuits which may be electrically connected to each other on one side of a base board, wherein the electrically conductive copper paste is coated on a part of a first copper circuit layer which is formed on one side of the base board and is to be electrically connected to a second copper circuit layer to be superposed on part of the first copper circuit layer, and then the electrically conductive copper paste is heated to be hardened, and then the hardened paste is chemically coated with a metal coating such as a copper coating so as to increase the electric conductivity of the electrically conductive copper paste up to that of the copper coating. Thus it becomes possible to provide at least two circuit layers on one side of a base board in contrast to the conventional base board having at least two circuit layers, one of which is provided on one side thereof and the other of which is provided on the other side thereof and electrically connected to each other by means of holes formed as extending through the base board. Further it becomes possible to eliminate the conventional processes for attaching the copper laminations on the base board and etching the laminations, to thereby drastically reduce the processing steps as well as the amount of material required for the processing steps approximately up to a half. Further it becomes possible to eliminate the conventional processing step for forming a hole or holes extended through the base board by means of the NC device which is considerably more expensive to purchase for that purpose, to thereby drastically simplify the processing of the base board, and to simultaneously reduce the cost of the completely processed base board approximately up to a half of the conventional one. It is another object of the invention to provide a multilayer print circuit board of a size reduced approximately up to a half of the conventional one by forming at least two circuit layers on both sides of a base board respectively which may be electrically connected by holes formed as extending through the base board. It is still another object of the invention to use an electrically conductive copper paste especially receptive to a metal plating so as to dispense with a processing step for activating the paste by means of a catalyst which is otherwise required before application of a metal plating process, thereby to additionally simplify the formation of a printed circuit board. It is still another object of the invention to form a multilayer printed circuit board having the first and second circuit layers provided on one side thereof which are respectively formed with the specific electrically conductive copper paste and a metal coating without the need of a copper laminating process and the etching process, to thereby considerably reduce the production process and the amount of material and accordingly the production cost.
In short, the present invention comprises the steps of: forming a first electrically conductive circuit as a first circuit layer on one side of a base board, the first circuit layer being receptive to a metal plating; coating a plating-resistant resist on the same side of the base board in a pattern excluding certain portions of the first circuit layer which are required to be electrically connected to another circuit layer to be subsequently formed on the first circuit layer; coating an electrically conductive copper paste to which a metal plating will adhere on the same side of the base board in a manner as to electrically connect said portions of the first circuit layer, however excluding certain regions of said portions of the first circuit layer which are to be subsequently processed with a metal plating; heating the base board to harden the same; cleansing the base board; and immersing the base board in a metal plating solution to form a metal plating on the electrically conductive copper paste and on said regions of said portions of the first circuit layer, to thereby form a second electrically conductive circuit lamination of said conductive copper paste and said metal plating on the first circuit layer.
Further the present invention comprises the steps of: attaching a copper layer on one side of a base board; etching the copper layer to form a first electrically conductive circuit layer; coating a plating-resistant resist on the same side of the base board excluding certain portions of the first circuit layer which are required to be electrically connected to another circuit layer to be subsequently formed on the first circuit layer; coating an electrically conductive copper paste receptive to a metal plating on the same side of the base board in a manner as to electrically connect said portions of the first circuit layer, with the exception of certain regions of said portions of the first circuit which are to be subsequently processed with a metal plating; heating the base board to harden the same; cleansing the base board; and immersing the base board in a metal plating solution to form a metal plating on the electrically conductive copper paste and on said regions of said portions of the first circuit layer, to thereby form a second electrically conductive circuit lamination of said conductive paste and said metal plating the first circuit layer, thus to provide at least two circuit layers electrically connected to each other on one side of the base board.
Further the present invention comprises the steps of: coating an electrically conductive copper paste being receptive to a metal plating on one side of a base board; heating the base board to harden the same; cleansing the base board; immersing the base board in a metal plating solution to form a metal plating on the electrically conductive copper paste, to thereby form a first electrically conductive circuit lamination of said conductive paste and said metal plating; coating a plating-resistant resist on the same side of the base board excluding certain portions of the first circuit lamination which are required to be electrically connected to another circuit lamination to be subsequently formed on the first circuit lamination; coating said electrically conductive copper paste on the same side of the base board in a manner as to electrically connect said portions of the first circuit lamination, with the exception of certain regions of said portions of the first circuit lamination which are to be subsequently processed with a metal plating; heating the base board to harden the same; cleansing the base board; and immersing the base board in a metal plating solution to form a metal plating on the electrically conductive copper paste and on said regions of said portions of the first circuit lamination, to thereby form a second electrically conductive circuit lamination of said conductive paste and said metal plating, thus to provide at least two circuit laminations electrically connected to each other on one side of the base board.